In 1987, C. W. Tang and VanSlyke in Eastman Kodak, an American company, reported a breakthrough in the search for organic electroluminescence. A double layer OLED having high brightness and high efficiency was successfully prepared by using ultrathin film technique. In such double layer device, the brightness can reach 1000 cd/m2 at 10V, and its luminous efficiency was 1.51 lm/W. The device lifetime was more than 100 hours.
OLED works on the principle that: in the case of applied external field, electrons are injected from cathode into the lowest unoccupied molecular orbital (LUMO) of organic matters, while holes are injected from anode into highest occupied molecular orbital (HOMO) of organic matters. The electrons and the holes move towards each other and they recombine, forming excitons in luminescent layer. Such excitons migrate in the electric field, and energy transfer to luminescent material. Electrons are excited and jump from the ground state to an excited state. The decay of this excited state results in a radiative relaxation of the energy levels of the electron, accompanied by release of the energy as photon.
In some traditional OLEDs, luminous efficiency is only about 18%. Most of the light is lost due to the factors like absorption and total reflection. Bottom emitting structure is commonly used. Glass has a refractive index of 1.5, whereas ITO has a refractive index of 1.8. When the light emitted from the organic layer reaches the glass through ITO, total reflection occurs due to the difference in refractive index. In the same way, total reflection occurs whenever light is emitted from the glass to the air, resulting in reduction of luminous efficiency.